In this episode of The 100-Year Thinkers, Robert Hagstrom explains why modern portfolio theory pulled investors away from business analysis and toward portfolio math. In this episode, Hagstrom, Matt Zeigler and Bogumil Baranowski discuss Markowitz, beta, efficient markets, Warren Buffett, Charlie Munger, business-driven investing, owner earnings, benchmarks, and why thinking like a business owner changes how investors understand risk.

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Topics covered:

• Why Hagstrom thinks modern portfolio theory changed investing’s objective

• The difference between volatility, variance and real investment risk

• How Benjamin Graham and John Burr Williams framed risk around intrinsic value

• Why beta became the dominant shorthand for risk

• How the 1973-74 bear market helped institutionalize modern portfolio theory

• Why Berkshire preserved the business owner’s lens

• The “cathedral and casino” distinction between owning businesses and trading stocks

• Owner earnings, return on invested capital and cost of capital

• Why business owners often make better long-term equity investors

• Look-through earnings and building a “mini Berkshire”

• The difference between making money and beating a benchmark

• How benchmarks can distort investor behavior

• Why knowing yourself and your clients matters in portfolio construction

Timestamps:

00:00 Robert Hagstrom on why risk is not volatility

00:40 Business-driven investing vs portfolio math

02:42 How modern portfolio theory defined risk as variance

06:38 Graham’s margin of safety vs Markowitz’s definition of risk

09:44 Sharpe, beta and simplifying portfolio risk

12:51 Why the 1973-74 bear market helped MPT take over

16:20 Why MPT became institutionalized without proving it could beat the market

18:53 Buffett, Keynes and concentrated investors violating MPT

22:53 Stocks as businesses and Buffett’s cathedral vs casino

30:01 Business analysis, owner earnings and return above cost of capital

36:41 Look-through earnings and running a mini Berkshire

41:34 Making money vs outperforming a benchmark

47:30 Why Berkshire’s public and private businesses shaped Buffett

50:05 How investors can start applying the Buffett way
54:05 Bogumil on how investing theory becomes accepted truth
58:09 Why direct ownership creates responsibility and conviction
01:00:15 Investor know thyself and the limits of outsourcing caring
01:03:35 Finding the right clients for a business-owner investing approach